1) Field of the Invention
The present invention relates to a ferroelectric element having a ferroelectric layer, and a method of manufacturing a ferroelectric element.
2) Description of the Related Art
A memory is widely used in an information processing apparatus including a computer and the memory includes a volatile memory and a nonvolatile memory. The volatile memory includes a semiconductor memory such as a DRAM and an SRAM. When a power supply is turned off, information stored in the volatile memory is lost. The nonvolatile memory includes a magnetic disk unit, an EEPROM, a flash memory, and the like. The nonvolatile memory has an advantage in that information stored in this memory is not lost even when a power supply is turned off. Recently, a ferroelectric element (FRAM: “FRAM” is a registered trademark) that stores information in a form of a spontaneous polarization of a ferroelectric layer is developed as this kind of nonvolatile memory. The ferroelectric element has a capacitor of a memory cell area structured by a ferroelectric. The ferroelectric element writes information using a polarization of the ferroelectric by applying an electric field, thereby increasing a writing speed and decreasing power consumption, as compared with an EEPROM and a flash memory that writes information by injecting electron. This ferroelectric element also has an advantage in that the life of the ferroelectric element can be increased by using the polarization of the ferroelectric.
However, the ferroelectric element is required to have an increased memory capacity and is required to operate stably even when the operation voltage is decreased from 5 V to 3 V to suppress power consumption.
The ferroelectric element involves a phenomenon that the polarization decreases while an inverted operation of the ferroelectric is repeated. This phenomenon is called fatigue characteristics, which impart a large influence on the life of the ferroelectric element. Therefore, the ferroelectric element is also required to have excellent fatigue characteristics.
As means for increasing the memory capacity, the size of the capacitor of the ferroelectric element can be made smaller. To decrease the capacitor means to decrease the amount of the ferroelectric. In other words, the switching charge per one capacitor becomes smaller. The ferroelectric element records information by changing over the switching charge of the ferroelectric. When the switching charge becomes smaller, the recorded information becomes unstable.
Further, as means for decreasing the operation voltage, the ferroelectric layer of the ferroelectric element is made thinner, and a large electric field is applied to this layer, thereby maintaining a large switching charge. However, when the thickness of the ferroelectric layer is decreased to 100 nm or below, a leak current increases rapidly. Therefore, there is a limit to the thinning of the ferroelectric layer. When the operation voltage is decreased, the fatigue characteristics can be decreased.
In order to solve these problems, Japanese Patent Application Laid-Open No. H10-335596 discloses a ferroelectric element having many ferroelectric layers sandwiched by conductive electrodes. In this ferroelectric element, a layer that is in contact with the conductive electrodes is a cubic system or a tetragonal system in which a ratio of a length of an axis c to a length of an axis a (c/a) of a crystal structure is equal to or larger than 1.00 and smaller than 1.02. A layer that is not in contact with the conductive electrodes is a ferroelectric layer of a tetragonal system. Japanese Patent Application Laid-Open No. 2001-181094 discloses a ferroelectric multilayer structure that includes at least a ferroelectric layer of a perobskite structure having [111] oriented grains and a film that gives a two-dimensional tensile stress within a surface perpendicular to the [111] direction of the ferroelectric layer, as a layer structure. Japanese Patent Application Laid-Open No. 2002-525876 discloses a ferroelectric memory cell including: a first metal oxide electrode; a ferroelectric layer that is formed on the first electrode, has a perobskite crystal structure, and includes Pb, Zr, Ti, O, and at least one additional rare-earth element; a second electrode that is formed on the ferroelectric layer; and a circuit that is connected to the two electrodes to feed power, control, and read a charge accumulated on the ferroelectric layer, and that has a maximum DC power supply voltage of 3 V or less, wherein the ferroelectric layer includes at least one of the rare-earth elements having a sufficient percentage, and the circuit can be operated at the maximum DC power supply voltage.
However, according to Japanese Patent Application Laid-Open No. H10-335596 and Japanese Patent Application Laid-Open No. 2002-525876, residual polarization is not large, and a switching charge Qsw is not large. According to Japanese Patent Application Laid-Open No. 2001-181094, the ferroelectric layer sandwiched by the electrodes has many layers, and a manufacturing process becomes complex. Therefore, none one of these disclosed techniques provide a satisfactory ferroelectric element that has a low operation voltage and a low power consumption that is excellent in fatigue characteristics, and that has a large switching charge Qsw.